Frequency measuring system



Jan. 30, 1951 Filed April 20, 1944 H. O. PETERSON FREQUENCY MEASURINGSYSTEM 5 Sheefcs-Sheet l ATTORN EY Jan. 30, 1951 H. `o. PETERSONFREQUENCY MEASURING SYSTEM BY )Rg/WM ATTORNEY Jan. 30, 1951 H. o.PETERSON 2,539,673

FREQUENCY MEASURING SYSTEM 5 Sheets-Sheet 5 T 1 El vff/v/w Filed April20, 1944 ATTORN EY Jan. 30, 1951 H. o. PETERSON FREQUENCY MEASURINGSYSTEM 5 Sheets-Sheet 4 Filed April 2o, 1944 INVENTOR //f/Pam ff/Pfam BYMr/JA,

. ATTORNEY Jan. 30, 1951 H. o. PETERSON 2,539,673

FREQUENCY MEASURING SYSTEM Filed April 20, 1944 5 Sheets-Sheet 5 'Tlc'-'7.

ATTORNEY Patented Jan. 30, 1951 FREQUENCY MEASURING SYSTEM Harold 0.Peterson, Riverhead, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application April 20, 1944, Serial No. 531,896

(Cl. Z50-39) 8 Claims.

The present invention relates to frequency measuring systems and, moreparticularly, to a system whereby the frequency of the radio frequencywave may be determined with extreme accuracy.

An object of the present invention is the measurement of the frequencyof radio frequency waves. n

A further object of the present invention is the provision of a radiofrequency measuring system which is so arranged that the frequency ofthe wave may be directly read from calibrations on the device.

A further object is the provision of a frequency measuring system whichhas an accuracy better than one part in ten million.

The foregoing objects, and others which may appear from the followingdetailed description, are attained by providing a high ,speed electroniccounter-mechanism which counts the number of cycles of radio frequencyenergy in an accurately determined time period.

The radio frequency wave whose frequency is to be determined may eitherbe directly applied to the counter or it may be beat against anotherwave of accurately known frequency and the frequency of the resultantbeat wave accurately counted by the electronic counter mechanism. In thesecond instance, the accurately known wave may be obtained by selectingcertain decimally related harmonics of a wave from a standard frequencysource and combining these harmonies to provide an accurately knownfrequency.

The present invention will be more completely understood by reference tothe following detailed description which is accompanied by a drawing inwhich:

Figure 1 illustrates in block diagram form an embodiment of the presentinvention;

Figure 2 illustrates in block diagram form a modification of the presentinvention;

Figures 3 and 4 illustrate in block diagram form further modificationsof the present invention;

Figure 5 illustrates a form of interval timer useful in practicing thepresent invention;

Figure 6 is a family of curves illustrating the operation of a portionof the device shown in Figure 5; and

Figure '7 illustrates a further modification of the invention.

In Figure l there is shown, generally, one system of measuring thefrequency of a signal wave. The incoming signal is picked by by aerialIl) and -frequency wave into the receiver l2.

applied to the input of receiver l2. frequency local oscillator lil alsofeeds a radio The frequency of the variable frequency oscillator i4 isadjusted to exactly the same frequency as that of the incoming signalwave. This adjustment to equality may be made in a well known manner bythe assistance of earphones l5, cathode ray oscilloscope i6, or otherconventional beat indieating means. l

The output of the variable frequency oscillator M. is also arranged tohe transmitted through an interval timing device i il, the output ofwhich is applied to a counting device 26. The Ycounting device must beone which operates at very high speeds. A known type of counting deviceoperates at such speeds that it actually counts the number of cycles ofradio frequency energy in a given interval of time, `and operates on theprinciple that each incoming cycle causes an increment of electriccharge to be added to a first condenser.

Circuits are so arranged that after ten such incremented charges havebeen added to the first condenser, means are actuated to supply ya unitcharge to a second condenser. At the same time, the accumulated chargeon the first condenser is dissipated, thus conditioning it for arepetition of the described action. The second device after receivingten charges actuates a third device to supply a unit charge to a thirdcondenser, and so on, until a low enough frequency of operation isobtained that it may be followed by the conventional mechanicalcounters.

Preferably, the counter deviceis so constructed that as each charge isadded to one of the charge maintaining condensers, one of a row of 9glow tubes associated with each condenser is illuminated. The rows ofglow tubes may be arranged in vertical rows like an adding machinekeyboard so that if, for example. eight charges are applied to onecondenser, eight of the glow tubes associated with that condenser willbe illuminated. Then as the charge on the condenser is dissipated, allof the glow tubes associated therewith are extinguished. By the use ofthis type of counter device the number of cycles of input energy in aVknown interval of time may be accurately counted, thus giving anaccurate determination of the frequency of the variable frequencyoscillator I4.

It is believed that the foregoing description of a satisfactory type ofcounter circuit is sufficient for a clear understanding of the presentinvention. A more complete disclosure may be had by A variable referenceto application Serial No. 459,404, filed September 23, 1942, by L. E.Flory and G. A. Morton, now U. S. Patent 2,442,403 issued June 1, 1948,and application Serial No. 467,032, led November 26, 1942, by L. E.Flory, now U. S. Patent 2,410,156 issued November 29, 1946.

The interval timer' is preferably constructed somewhat similarly to thecounter mechanism, and is so arranged as to close the circuit betweenoscillator it and counter 20, count off a predetermined number of cyclesof energy from the standard frequency source 22 and then open thecircuit between variable frequency oscillator l and counter 20. Forexample, if the standard frequency source 22 is a 100 kilocyclegenerator, the interval timer in measuring one second of time, afterhaving closed the counting circuit, may count off 100,000 cycles fromthe frequency standard 22 and then open the counting circuit. Then thereading of counter 20 gives directly the frequency of oscillator I4 toone part in 100,000.

If desired, a harmonic of the variable fre quency oscillator I4 may beused for matching the unknown frequency applied to antenna l0. Thefundamental frequency of the variable frequency oscillator lli may be,as before, transmitted through the interval timer. In that case thefrequency would be determined by multiplying the measured frequency lofthe variable frequency oscillator by the known order of the harmonicused.

Figure 2 shows a modification of the present invention utilizing theinterval timer and counter device of Figure l in conjunction with thevvariable frequency generator system described in my prior applicationSerial No. 489,085, filed May 29, 1943, now U. S. Patent 2,380,868,issued July 17, 1945.

In the arrangement of Figure 2, there is shown a 100 kilocycle persecond standard frequency source 22. The output from the 100 lio/sec.standard source is applied to multiplier 25, the output of which is at afrequency of one megacycle per second. The one megacycle per second waveis applied to a harmonic generator 28 having output frequencies at everymegacycle from 9 to 18 mc., inclusive. The output from the harmonicgenerator 28 is applied to a bank of harmonic selectors and mixers 30.This bank of selectors and mixers is here shown as a single blockinstead of as a plurality of separate circuits in my prior applicationSerial No. 489,085, now U. S. Patent 2,380,868, issued July 31, 1945,and to which reference may be had for a more complete description of theoperation and function of the harmonic selectors and mixers.

Briefly, the function of the bank of selectors and mixers 30 is toselect one of the harmonics generated by generator 28 and mix anincoming wave from a source to be later described with this selectedharmonic. A further selection takes place after the mixing to selectonly the desired one of the products of the mixing. Thus, if forexample, the 9 mc. harmonic from harmonic gen erator 28 is mixed with aone mc. wave .from the other source to be later described, the outputfrequency selected will be 10 mc. This 10 inc. wave is applied toreceiver i2 where it is coinbined with the incoming signal from antennai0. An indication of the beat obtained between these two waves isobserved by means of phones i or oscilloscope l5.

The 100 kc. standard source 22 has a second output directly to harmonicgenerator 3i which generates harmonics from .9v to 1.8 mc.. A Secondbank 32 of harmonic selectors and mixers is connected to the output ofharmonic generator 3i whereby any one of the harmonics generated by thegenerator 3l may be mixed with an incoming wave from variable frequencyoscillator 33 to provide a definitely known and accurately determinedfrequency in the range of from l to 2 mc. This frequency is yapplied tothe first bank of harmonics, selectors and mixers 38. The variablefrequency oscillator 33 also has an output through interval timer I8 tocounter 20. The interval timer I8 is actuated from the 100 kc. standardsource 22 as described with reference to Figure l.

In operating the modification of Figure 2, the output of variablefrequency oscillator 33 is combined with two decimally related selectedhar'- monics of the local frequency standard source 22 to produce afrequency which, when beat with the incoming frequency in receiver i2,provides a zero beat indication at the head phones l5 or oscilloscopei0. The actual frequency generated by the variable frequency oscillator33 is determined by counting the number of cycies of the wave in anaccurately predetermined time interval such as one second, through theuse of interval timer i8 and counter 20.

A further modification of the present invention is shown in Figure 3wherein a harmonic of a variable frequency .oscillator v35 is matchedagainst the unknown frequency. Since a har* monic of the variablefrequency is used, the oscillator is not required to cover a largef-requency range. The output of variable frequency oscillator 36 ismixed with a constant frequency from a standard frequency source 22 in aconverter 38 so as to produce a beat note which is applied through timerI8 to counter 20, as outlined above.

The output of variable frequency oscillator `36 is also applied to aharmonic generator 3l, and one of the resultant harmonics is combined inreceiver l2 with the incoming frequency picked up by antenne. l0. Whichof the harmonics is used may be readily determined by even a roughcalibration of receiver i2. Using a structure as shown in Figure .3, itis possible to measure any frequency in the range between 1 mc. and 20mcs. by using the proper harmonics from variable frequency generator 36,said generator being variable in frequency only over a band of 10,000cycles.

The unknown frequency is determined by multiplying the frequency of thevariable frequency oscillator 36 (which is accurately determined bycounter 29) by the order of the harmonic used. In some cases it may beconvenient to have more than one frequency range in the variablefrequency oscillator 30. For example, thevariable frequency oscillator36 may cover the ranges of l0 to 11 kc., 100 to 110 kc. and l to 1.1 mc.With the switching of the ranges in the variable fren quency oscillator36, means may also be provided for switching the other branch of theconverter input 33 to dierent outputs of the local frequency standard22, that is, for example, to l0, kc. and l mc. output.

A further modification of the present invention is shown in Figure 4wherein the same system of obtaining and mixing accurately knownharmonies of a standard frequency source 22 is employed to obtain anaccurately known local wave. The process has been set forth in somedetail with reference to Figure 2 and will not, therefore. be completelydescribed with reference to Figure 4;

However, in the present modification the outputfrom standard` frequencysource 22 is additionally applied to a divider 43 to obtain a 10 kc.wave. The output from divider 43 is app-lied to a harmonic generator 44,generating harmonics at 10 kc. intervals from 100 to 200 kc. One ofthese harmonics may be selected by harmonic selector 45 for applicationto the first bank of harmonic selectors and mixers 32 `for mixing with aselected harmonic from generator 3|.

It will be noted by comparing Figures 2 and 4 that the frequency divider43, harmonic generator 44 and harmonic selector 45 are substituted forthe calibrated oscillator 33 of Figure 2. Selection at 45, 32 and 36 ofappropriate harmonics provides, in effect, a frequency source supplyingto an input of mixer 46 accurately known frequencies in steps of 10 kc.over the entire band from 10 to 20 mc. One of these frequencies, whenmixed in mixer 46 with the incoming signal from receiver I2, produces anaudible beat indication in phones I5 since there is only 10 kc.difference between any pair of adjacent frequencies. The audible beatnote is applied through interval timer |8 to counter 20 and may readilybe measured. Since there will always be at least two frequencies whichproduce an audible beat which may be heard in the head phones, acomparison of the results of mixing a frequency which is higher or lowerthan the incoming waves avoids any ambiguity as to whether the frequencyindicated by counter 2I is to be added to or subtracted from the readinggiven by the calibrated selector Switches on selectors 30, 32 and 45.

In any of the previously described modifications, if it is desired tocover frequencies outside of the range from 10 to 20 mc., which has beendiscussed for purposes of illustration, the range may be extended, asdescribed in my previous application` Serial No. 489,085, now Patent2,380,868 by mixing with the output of the bank of harmonic selectorsand mixers 3U, for example, a l mc. wave. By choosing the proper one ofthe sum or difference frequencies resulting from this additional mixing,the range may be extended to cover 1 to 10 mc. or 20 to 30 mc. The rangemay be further extended by utilizing harmonics of the 1 to 30 mc. rangeso far described.

The interval timer shown in Figure 5 consists of a gate circuit throughwhich the unknown frequency is passed for a known length of time. InFigure 5 the gate is represented by vacuum tubes 62 and 63 with theirassociated connections. The numb-er of cycles of the unknown frequencypassed by the gate in a known length of time is indicated on theregister of the counter 65.

The gate circuit is characterized in that the mutual conductance of tube62is varied to either permit or prevent the passage of the unknownfrequency, while tube 53 acts to maintain the anode current constantthrough resistor III). The anode of tube 62 and the screen of tube 63are connected together, while the screen of tube 62 and the anode oftube 63 are supplied from B+ through resistors and ||2, respectively.When a negative potential is applied to both suppressor grids, theconductance of tube 62 is reduced and the anode voltage tends to rise bythe reduction of the current thro-ugh resistor Illl. But the currentthrough resistor H2 is similarly reduced, the difference in currentpassing to the screen of tube 63, exactly replacing the decrease throughI I6 due to the reduction in anode current of tube 52. If the suppressorgrids are so biased as to vpermit tube 62 to act as an amplifier, the

resistor ||0 in the usual way.

The interval timer shown in Figure 5 measures an interval of time bycounting a predetermined number of cycles of a standard frequency. Inthe example given, the standard frequency is cycles which is derivedfrom the 100 kc. frequency standard 22 by means of frequency dividers52. This process is well known in the art.

The 100 cycle standard frequency is passed through a current limiter 53which results in an output wave form such as shown by curve 54 in Figure6. When current having this type of Wave form is passed through theprimary of transformer 55 and transformer 56, there will be induced asecondary voltage having the wave form shown at 51 in Figure 6. It willbe noted that the secondary voltage shown at 56 in Figure 6 consists ofshort pulses, alternately positive and negative. If transformers 55 and56 are properly designed, thesepulses can be made to have a length ofabout 1 microsecond.

Vacuum tubes 58 and 59 are biased negative so that they normally do nottransmit plate current. The cut-off bias is provided in part by grid`biasing batteries 68, and in part by a negative bias applied to thesuppressor grids. In the case of tube 58, the suppressor grid bias issupplied from battery 16, while the suppressor grid bias for tube 59 isprovided by a connection to the circuit involving tubes 66 and 61.

Tubes 66 and 61 are connected in a flip-flop circuit. Tubes 68 and 69are also connected in a flip-flop circuit. This flip-flop circuit hasthe characteristic that either one of the two tubes may conduct current,but while it is conducting current the voltage drop in its plate loadcircuit biases the other tube to cut-off. That is, if it is assumed thattube 66, for example, is conducting, there will be a voltage dropthrough its plate resistor |04. The grid of tube 61 is connected to avoltage divider |08, |09, from the plate end of resistor |04 to ground.The constants are so arranged that the resultant bias on tube 61 biasesit to cut-off.

Now, if a short pulse of voltage in the proper direction is applied tothe grid of tube 66, the other tube will be caused to draw plate currentthrough resistor |05 and bias tube 66 to cut-off through a voltagedivider arrangement 202, 263 similar to |08, |89. Since the flip-flopcircuits involving tubes 68 and 69 operate in a similar manner, thecircuit elements involved with these tubes have not 4been separatelyidentified by reference numerals.

We will assume that th" circuits are in a condition which closes thegate to the unknown frequency. In this condition, vacuum tube |56 isdrawing plate current which causes tubes 68, 62, 63 and 59 to be biasedto cut-off, Vacuum tube 58 is receiving positive pulses, such as shownat 51 in Figure 6, on its grid, but no plate current flows because ofthe negative cut-off voltage applied to the suppressor by battery 1|).Now, if the operator closes contactor 1| for a brief instant, thenegative bias is removed from the suppressor and an impulse of platecurrent will be drawn when the next positive pulse is applied to thecontrol grid by transformer 55. This pulse of plate current flowingthrough resistor 12 results in a` negative pulse being transmittedthrough capacitor 13 to the control grid of tube 6E. This is sufficientto cause the flip-flop circuit to flip over to the condition wherevecu-'- um tube 61 is drawing plate current andV tube 66 is biased tocut-olf. When this has happened, the suppressor grids of tubes 59, 52and 63 are no longer biased to cut-off. Thus the unknown frequency isallowed to pass by the gate tubes 52 and 63, and pulses are transmittedby tube E@ into the counter chain 'F4 and l5. This process continues aslong as vacuum tube 61 continues to draw current, which will be until itreceives a pulse of the proper polarity to shut it off.

We will also assume that while the interval timing procedure gets underway, vacuum tube 68 is drawing plate current which biases the suppressorof tube El to cut-olf. Now, every time counter lli reaches a count of10, an impulse is transmitted into counter l5. Counter 75 counts theimpulses which it receives. Thus, when counter reaches a count of 9, itmeans that 9G pulses have been counted by counter 74. When counter lreaches a count of 9 it causes a negative pulse to be transmittedthrough condenser 'I6 which stops the flow of plate current in tube B8,causing the circuit to flip over sc that tube 69 draws plate current.This removes the negative cut-off bias from the suppressor of tube el sothat when counter M next reaches a count of 10 its pulse transmittedthrough condenser' 'il will cause a negative pulse in the plate circuit13 which is transmitted through coupling condensers 79 and 80. Thisnegative pulse causes another reversal of the two dip-flop circuits sothat tubes 61 and 69 immediately cease drawing plate current, and tubes66 and 68 simultaneously commence drawing plate current which causes theSuppressors of tubes S2, t3 and 59 to be again biased to cut-off. Thus,the gate has been opened for a period of time accurately determined bythe counting of a predetermined number of cycles of a known frequency.

A slightly different way of determining the unknown frequency is shownin Figure 7. In this case, two gates are provided by the circuitsassociatcd with tubes 8l, 82 and 33, 8f3. These crate in the same way asdescribed heretofore with reference to tubes 62, 53, and the operationwill not therefore be again described, nor is it believed necessary toidentify in detail the circuit connections to these tubes. Both gatesare simultaneously opened by closing a "tch or telegraph key 7i. Openingswitch or key li closes both gates simultaneously, thus the standardfrequency is transmitted into counter 55 and the unknown frequency istransmitted into counter 2t for an equal interval of time. Since thestandard frequency is known, the unknown frequency readily calculatedfrom the readings cf the two counters. The unknown frequency would beequal to the standard frequency multiplied by the reading of counter 2Sdivided by the reading of counter 65.

While I have illustrated a particular embodiment of the presentinvention, it should be clearlyr understood that it is not limitedthereto since many modifications may be made the several elementsemployed and in their arrangement without departing from the spirit andscope of the invention.

Having now described my invention, what I means to produce a differenceWave having a frequency equal to the difference in frequency betweensaid periodic and alternating waves, a counter arranged to count thealternations of said difference wave, and gating means coupled to saidsource tc apply said difference wave to said counter for a preciselydetermined time in- Y terval whereby the frequency of said alternatingwave is the algebraic sum of the number of alternations counted dividedby said time interval and the frequency of said periodic wave,

2. An arrangement for determining the frequency of an alternating waveincluding a single source of oscillations of precisely known frequency,means coupled to said source to produce a periodic wave of frequencyprecisely proportional to the frequency of said oscillations, means tomix said periodic wave and said alternating Wave to produce analternating current having a frequency equal to the difference infrequency between said waves, a counter arranged to count thealternations of said alternating current, and switching means coupled tosaid source to couple said counter to said mixer for a preciselydetermined time interval whereby the frequency of said alternating waveis the algebraic sum of the number of alternations counted divided byYsaid time interval and the frequency of said periodic wave.

3. An arrangement for determining the frequency of an alternating waveincluding a single source of oscillations of precisely fixed frequency,means coupled to said source to produce a periodic wave of frequencyintegrally proportional to the frequency of said oscillations, a mixer,means to apply said periodic wave and said a1- ternating wave to saidmixer to produce an alternating current having a frequency equal to thedierence in frequency between said waves, a counter arranged to countthe alternations of said a-ternating current, and gating means coupledto said source to couple said counter to said mixer for a preciselydetermined time interval whereby the frequency of said alternating wave,i is the algebraic sum of the number of alternations counted divided bysaid time interval and the frequency of said periodic wave.

4. An arrangement for determining the frequency of an alternating Waveincluding a single source of oscillations of precisely fixed frequency,harmonic generators having decimally related output characteristicscoupled to said source to produce a periodic wave of frequency decimallyproportional to the frequency of said oscillations and lower than thefrequency of said alternating wave, a mixer, means to apply saidperiodic wave and said alternating wave to said mixer to produce analternating current having a frequency equal to the difference infrequency between said waves, a counter arranged to count thealternations of said alternating current, and gating means coupled tosaid source to couple said counter to said mixer for a preciselydetermined time interval whereby the frequency of said alternating Waveis the arithmetic sum of the number of alternations counted divided bysaid time interval and the frequency of said periodic wave.

5. An arrangement for determining the frequency of an alternating waveincluding a single source of oscillations of precisely xed frequency,means coupled to said source to produce a periodic Wave of frequencyprecisely proportional to the frequency of said oscillations, a mixer,means to apply said periodic Wave and said alternating wave to saidmixer to produce an alternating current having a frequency equal to thedifference in frequency between said waves, a counter arranged to countthe alternations of said alternating current, a gating circuit havingcounting means therein coupled to said source to count a predeterminednumber of cycles of said oscillations to precisely determine a timeinterval and further means responsive to said counting means to couplesaid counter to said mixing circuit for said time interval, whereby thefrequency of said alternating wave is the algebraic sum of the number ofalternations counted divided by said time interval and the frequency ofsaid periodic wave.

6. The method of determining the frequency of an alternating Waveincluding the steps of producing oscillations of precisely knownfrequency, producing a periodic Wave having a frequency preciselyproportional to the frequency of said oscillations and of the order ofthe frequency of said alternating wave, mixing said alternating wave andsaid periodic wave to produce an alternating current having a frequencyequal to the difference in frequency between said waves, determining atime interval proportional to the period of said oscillations, anddetermining the number of cycles of said alternating current occurringduring said time interval, whereby the frequency of said alternatingwave is the algebraic sum of said number of cycles divided by said timeinterval and the frequency of said periodic wave.v

'7. The method of determining the frequency of an alternating waveincluding the steps of producing oscillations of precisely knownfrequency, producing a periodic wave having a frequency preciselyproportional to the frequency of said oscillations and of the order ofthe frequency of said alternating wave, producing a difference wavehaving a frequency equal to the difference in frequency between saidalternating and said periodic waves, determining a time intervalintegrally proportional to the period of said oscillations, anddetermining the number of cycles of said diiference wave occurringduring said time interval, whereby the frequency of said 10 alternatingwave is the algebraic sum of said number of cycles divided by said timeinterval and the frequency of said periodic wave.

8. The method of determining the frequency of an alternating Waveincluding the steps of producing oscillations of precisely knownfrequency, producing a periodic Wave having a frequency preciselyproportional to the frequency of said oscillations and of the order ofthe frequency of said alternating wave, generating a further wave havinga frequency decimally related to said periodic wave, combining saidperiodic and said further waves to produce an exact periodic wave,beating said exact periodic wave against said alternating wave toproduce an alternating current having a frequency equal to thedifference in frequency between said alternating wave and said exactperiodic wave, determining a time interval integrally proportional tothe period of said oscillations, and determining the number of cycles ofsaid alternating current occurring during said time interval, wherebythe frequency of said alternating Wave is the algebraic sum of saidnumber of cycles divided by said time interval and the frequency of saidperiodic wave.

HAROLD O. PETERSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,966,230 Andrew July 10, 19341,982,340 Forbes Nov. 22, 1934 2,019,503 Page Nov. 5, 1935 2,186,182Stocker et al Jan. 9, 1940 2,321,315 Peterson et al. June 8, 19432,380,288 Bligh July 10, 1945 2,405,597 Miller Aug. 13, 1946 FOREIGNPATENTS Number Country Date 355,705 Great Britain Aug. 24, 1931

